Method for preparing sources of ionizing radiation



Jan. 22, 1963 J. H. COLEMAN 3,074,811

METHOD FOR PREPARING SOURCES OF IONIZING RADIATION Filed April 22, 1957 2 Sheets-Sheet 1 INVENTOR 2 E JOHN H. COLEMAN Ema/W ATTORNEY Jan. 22, 1963 J. H. COLEMAN 3,074,811

METHOD FOR PREPARING SOURCES OF IONIZING RADIATION Filed April 22, 1957 2 Shasta-Sheet 2 Z w W9 //0 04 f fly //4 I /Z w U67 w m INVENTOR 4 JOHN H. COLEMAN ATTORNEY United States Patent 3,074,811 METHOD FOR PREPARING SOURCES OF IONIZING RADHATION John H. Coleman, Palm Beach, Fla, assignor to Radiation Research Corporation, a corporation of Florida Filed Apr. 22, 1957, Ser. No. 654,360 11 Claims. (Cl. 11767) This invention relates to new and improved methods for making bodies having metal coatings containing a sorbed gas. More specifically, the invention relates to new and improved methods for making metals clad with tritiated films. While the discussion of the invention which follows relates to the preparation of metals clad with tritiated films, it will be understood that the methods have utility in the preparation of bodies clad with metals containing other gases.

In the past, advantage has been taken of the high afiinity of metals such as lithium, tantalum, beryllium, and zirconium for gases such as oxygen, nitrogen, hydrogen, deuterium, and tritium, for utilization, for example, as targets under bombardment by high energy particles in particle accelerators. Bodie clad with a coating containing tritium also have utility as sources of ionizing radiation for the excitation of luminescence in phosphors, for production of positive or negative ions, or for production of beta particles for use in direct conversion of nuclear electric generators such as are disclosed in my copending United States patent application, Serial No. 543Jl08, entitled Nuclear Electric Generator, filed on October 27, 1955, now abandoned.

Tritium, the radioactive isotope hydrogen 3, is characterized by the emisison of beta particles having a maximum energy of about 18,000 electron volts and a mean energy of about 6,000 electron volts. Because of the high cost of tritium and the inability of its radiations to penetrate substantial thicknesses of a containing material, sources of ionizing radiation in which tritium is employed must be extremely thin in order to avoid waste. In fact, it is well known that beta particles of maximum energy emitted by decaying tritium will not penetrate a layer of aluminum, for example, with a thickness greater than 0.54 milligram per square centimeter. Therefore, if the metal containing tritium is titanium, this maximum range for electrons from layers farthest from the free surface corresponds to a thickness of about 1.2. microns. In the past, small areas of backing metal have been clad with tritiated films of other metals having thicknesses of this order of magnitude, but the methods and apparatus used for this purpose have been incapable of producing radiation sources of large area or of consistent thicknesses such as are required for many applications. It is an object of this invention to provide a method and apparatus capable of repeatedly producing large areas of metal clad with thin films of consistent thickness.

A second factor affecting the efficiency of tritium utilization is the amount of tritium contained in a film of a given thickness. Since hydrogen may be combined with titanium or zirconium, for example in combination ratios of 1.75 and 1.92 respectively, it may be expected that the isotope tritium will combine in similar ratios. However, wher a previously prepared film or layer of these metals is subsequently exposed to tritium gas, workers in the art have heretofroe had difficulty in producing unifomr combination ratios, and ratios consistently approaching full saturation. For example, one method previously employed produces films of tritiated zirconium having combination ratios of between .8 and 1.2, with only occasional ratios as high as 1.5 being achieved. For this reason, the source area required to produce a given amount of ionizing radiation has been unduly large. In

3,074,81 l Patented Jan. 22, 1963 some foils prepared according to the teachings of the prior art has varied widely from point to point over the surface of the foil, indicating non-uniform combination of tritium with the sorber metal, and preventing use of the foils, for example, as excitation sources for luminescent plaques of a substantial size and having uniform light output. It is a further object of the invention to provide an apparatus and method capable of producing foils of substantial area having maximum ionizing radiation per unit area and which, by the same token, have substantially uniform emission characteristics over large areas.

A third factor of importance in connection with the formation of highly tritiated foils is contamination of the sorber film. Contamination of the sorber film prior to sorption of the desired gas may result from inclusion of impurities in the sorber metal prior to its use or from contaminants introduced during preparation of the film. Such contaminants include gases such as nitrogen, oxygen, hydrogen, etc., which react readily with the sorber metals to form nitrides, oxides, hydrides, etc., as well as oil or water vapors which may similarly combine with sorber metal atoms to take space in the sorber film which could otherwise be occupied by the desired gas. In addition, layers of nitrides, oxides, or other impurities, formed on the surface of the sorber film impede the passage of the desired gas into the body of the film, and, since they cannot readily be reduced or removed, must be dispersed into the film in order to facilitate tritiation. It is another feature of the invention that contamination of the sorber film is substantially reduced. In fact, such layers of contamination as occur during the preparation of sorbing films in accordance with the teachings of the invention are so slight that newly formed titanium films, for example, will readily sorb tritium without the need for dispersal of surface contamination into the film body. Indeed, in the case of titanium films, it has been found that newly formed titanium films will absorb tritium gas quite readily at much lower pressures than would be expected from data reported in the literature, and with out the need for supplemental heat to assist in tritiation.

Still another object of the invention is the provision for producing metals clad with tritiated films which does not require the use of large quantities of tritium in excess of process needs, although the apparatus is adapted for such use, should the user so desire.

A still further object of the invention is the provision of a method for the production of materials clad with films containing a specific, occluded gas, which method is simple, reliably operable by unskilled personnel, and capable of repeated utilization to produce foils of uniform quality.

These objects and others which will appear from the below appended description are met in the invention which may be briefly described as follows. A radiation source of large area is formed in a reaction chamber on a large area substrate of thin sheet metal by evaporation from a filamentary heat source followed by sorption of the desired gas in the same chamber. In the preferred embodiment of the invention, the metal substrate or source backing is formed into a hollow cylinder and disposed concentric-ally about an elongated filamentary heater wire which carries a length of evenly distributed sorber metal of predetermined Weight. The cylinder of backing material and the axial filament are enclosed in a conformably fitting, .evacuable reaction chamber having a supporting structure of low thermal conductivity for the addition, the specific amount of radiation emitted by substrate. The backing material and the sorber metal are simultaneously outgassed under vacuum by radiant and conduction heat, respectively, from the heat source, and the sorber metal is then evaporated from the filament onto the substrate to form a film of predetermined, controlled thickness. The gas to be sorbed is admitted to the reactionchamber immediately upon completion of the evaporation step and is sorbed by the film. Supplemental heat may be supplied by radiation from the filament to initiate the reaction in the case of certain sorber metals or to diffuse surface film impurities into the sorbing film.

Alternatively, the same apparatus, modified to permit the separate application of voltage to filament and substrate, may be evacuated and filled with an inert gas at low pressure. A suitable voltage is impressed between the filament and the substrate to produce a gas discharge which cleans filament, charge and substrate surfaces by ionic bombardment. After bombardment, all gas is pumped out of the reaction chamber, and formation and tritiation of a sorber film on the substrate carried out as before.

In the drawings:

FIG. 1 shows a cross section of a tritiated foil produced according to the invention;

FIG. 2 schematically illustrates an apparatus for use in producing tritiated foils according to the teachings of the invention;

FIG. 3 is a view in cross section of one form of reaction chamber useful in producing tritiated foils according to the teachings of the invention; and

FIG. 4 is a view in cross section of a second form of reaction chamber according to the teachings of the invention.

Referring now to FIG. 1, it will be seen that tritiated foil or large area radiation source 2 produced in accordance with the teachings of the invention comprises a sheet or substrate of backing material 4, preferably metal, which supports a layer 6 of material with a high affinity for the desired gas such as tritium. Materials which have been found particularly useful as foil backirws are tungsten, silver, copper, molybdenum, nickel and stainless steel. Other materials may also be used provided that they are capable of forming a good bond with the layer of sorbing metal and provided further that they do not themselves have high affinity for the particular gas to be sorbed. High adherance of sorber material to backing material is required since the foil may be used, for example, as an unenclosed ionization source and under such conditions flaking of the radioactive layer is undesirable. Low sorption of the gas to be sorbed by the backing metal is desirable when costly gases such as tritium are employed. Of the materials listed, stainless steel possesses good working characteristics as far as adhesion, low tendency to embrittlement, low cost, and low occulusion of gases are concerned. The thickness of backing sheet 4 is not critical, inasmuch as it merely serves as a support for extremely thin source layer 6.

On the other hand, the thickness of sorber layer 6 must be carefully controlled. As indicated above, the etficiency of the radioactive source is a function of sorbing layer thickness. If the thickness of radioactive layer 6 is too great, escape of even the highest energy beta particles emitted for example, from a decaying tritium atom in the region of interface 7 between source layer 6 and substrate 4 is prevented, and no contribution to the total source emission is made.

For titanium, I have found that a sorbing layer thickness of approximately 1.2 microns is the maximum thickness which may be used without waste of tritium. For zirconium the maximum sorbing layer thickness is approximately .85 micron. Lesser thicknesses, while reducing the total emission per unit area of source, are superior for many applications, however, since they substantially reduce the cost of tritium required per unit area and assure adherence of all of the tritiated film to the substrate.

The radiation source of FIG. 1 and other such structures having a gas containing film upon a substrate may be produced according to the teachings of the invention by means of the apparatus of FIG. 2 in conjunction with a reaction chamber such as is shown in FIGS. 3 or 4.

The apparatus of FIG. 2 includes main evacuation line 9 which interconnects reaction chamber 26 and oil diflusion pump 10. Line 9, which is of a suflicient diameter to permit rapid evacuation of reaction chamber 26, contains valve 12 for isolating reaction chamber 26 from pump it). Pump 10 is connected to backing or fore pump 14 in accordance with the usual practise. A second, parallel passage from reaction chamber 26 to vacuum pump 10 is provided by bypass line 11 which connects to line 9 on either side of valve 12. Manifold line 8 is connected to the center of bypass line 11. Valves 13 and 15 are interposed on line 11 on either side of manifold line 8 and so permit connection of the manifold to reaction chamber 26 or vacuum pump it at will. Manifold line 3 provides valved access to low pressure gauge 16, which may be of the diaphragm actuated type, by means of valve 17, to vacuum gauge 13 by means of valve 19, to coupling 24 and gas storage flask 28 by means of valve 20, to uranium trap Si) by means of valve 32, and to auxiliary connection 36 by means of valve 34. For considerations of system volume discussed below, the length of line 9 from valve 12 to reaction chamber 26, the length and diameter of line 11 between valve 15 and reaction chamber 26, and the length and diameter of line 8 and its connected apparatus are kept as small as possible.

Electrical power for supplying heat to reaction chamber 25 is conveyed by means of electrical leads 2i and 22 from autotransformer 23. Autotransforrner 23 is connected, in turn, to terminals 25 and 2-7 for connection to alternating current of suitable characteristics. Heat for activating uranium trap 3% may be supplied by means of a suitable heater coil 46 connected through rheostat 42 to terminals *lectrical power of suitable current and voltage characteristics may be connected to terminals 44-. It will be understood that heat may be alternatively supplied to uranium trap 39 by means of a torch or other means well known in the art in lieu of heater coil 46.

With the exception of reaction chamber 26, all of the elements interconnected by the aforementioned system of piping are, individually, conventional in nature as will be understood by those skilled in the art.

Referring now to FIG. 3, one embodiment of reaction chamber 25 comprises hollow cylinder 46 having end plates 48, 5t} and 51. Cylinder 46 may be made of glass or some other electrically insulating material of suitable rigidity and vacuum quality. End plate 45} is provided with central aperture 52 into which exhaust pipe 54 is soldered. End plate 43 is thus permanently fastened to the system of FIG. 2, pipe 54 being represented schematically as 9 in FIG. 2. The inner surface of end plate 43 is also provided with annular recess 56 for receiving circular O ring 58. 0 ring 58 may be of rubber or other material well known in the art and provides a hermetically sealed joint between cylinder 46 and end plate 48. Radially disposed at intervals near the periphery of end plate 48 are three or more apertures 60 which receive flange electrical insulator sleeves 62. Electrical insulator sleeves 62, in turn, receive side bolts 64 and electrically separate them from end plate 48. End plate also carries U- shaped terminal bracket 66 having screw 78 for anchoring one end of tungsten filament '78 on the common axis of end plate 48 and cylinder 46.

Cooperating end plates 50 and 51 are mounted together to close the other end of cylinder 46 and are fixed in position relative to both of these elements by means of side bolts 64 passing through peripheral apertures 63 and 69, respectively. The assembly is held together by wing nuts 67 on the ends of side bolts 64 which bear against the outer surface of end plate 51. End plate 50 has inner annular recess 71 for receiving 0 ring 72, outer annular recess 74 for receiving 0 ring 76, and central aperture 77 for passage of filament 78. In order to facilitate evacuation, end plate 5-,; is further provided with apertures 80 within the region bounded by 0 ring groove 71. Filament 78, after passing through end plate 5i), passes axially through filament tensioning spring 86 into collar 88 where it is anchored by means of set screw 84.

End plate 51 is provided with annular recess 82 of a diameter equal to that of annular recess 74 for receiving 0 ring 76. The central portion of end plate 51 is deeply recessed on the side facing into reaction chamber 26 to provide clearance space for tension spring 86 and filament fastening collar 88. Provisions for electrical connection to the assembly are made by means of leads 21 and 22 fastened to end plates 48 and 51} respectively.

As will be seen from the drawings, the circular end faces of hollow cylindrical body 46 mate with 0 rings 58 and 72. 0 ring 76, in turn, seals the gap between coating and plates 50 and 51. In this Way, using conventional vacuum techniques, a hermetically enclosed reaction chamber (except for exhaust pipe 54) is formed for receiving and supporting hollow backing material cylinder 93 in coaxial, spaced relation with filament 78. For the purpose of providing a higher degree of thermal insulation than is provided by glass cylinder 46, a thin cylindrical layer 91 of glass wool, for example, may be inserted between cylinder 46 and work cylinder 93 The embodiment of reaction chamber 26 illustrated in FIG. 3 is particularly useful where the highest degree of thermal insulation of the sorber body backing metal is desired. The combination of low conductivity material in the wall of cylinder 46 and the added layer 91 of glass wool meets this need. In addition, the transparent wall of cylinder 46 permits ready observation of condition and temperature of the filament during processing. However, glass wool spacer 91 occludes gas, adds to the internal volume of the system, and the cut and ground ends of glass cylinder 46 have a tendency to chip, endangering the integrity of the vacuum. The reaction chamber structure illustrated in FIG. 4, overcomes these disadvantages and performs satisfactorily in production set-ups.

The embodiment of reaction chamber illustrated in FIG. 4 has reaction chamber cylinder 96, which may be of stainless steel, provided with integral end flanges 98 and 100. End flanges 98 and 108 have annular 0 ring recesses 104 on their surfaces for receiving 0 rings 1% and 108, and are also provided with circularly spaced holes 109 and 111, respectively, for receiving closure bolts 110. Holes 111 in flange 109 are enlarged to receive dielectric inserts 114 for electrically insulating bolts 110 from flange th.

Flange 98, which forms the end of the reaction chamber from which evacuation is accomplished, is also provided with inner peripheral lip 115 on which filament support disc 116 seats. Filament support disc 116, which may be of stainless steel, is provided with central aperture 118, for passage of heater filament 120, and with larger apertures 122 radially spaced between aperture 118 and the periphery of disc 116 to facilitate evacuation. Countersink 124 surrounds filament aperture 118 and receives one end of filament tensioning spring 126, preventing lateral slippage. Filament 120 thus passes axially through coil spring 126 into filament fastening collar 128. The axial filament receiving passage of collar 128 is intersected by a threaded hole for filament fastening set screw 130.

The end of the reaction chamber assembly discussed in the preceding paragraph, that is the end having flange 98, mates with evacuation end plate 134, being fastened thereto by means of the passage of closure bolts 110 through suitably located holes in disc 134. 'Hermetic closure of the joint thus made is accomplished by O ring 105. The central portion of end plate 134 is drilled out to receive exhaust connection pipe 54 which, in turn, is of sufiicient inside diameter to permit clearance of filament fastening collar 128 and spring 126. End plate 134 is permanently fastened to exhaust line 54 represented by line 9 in the exhaust system of FIG. 2.

Vacuum closure of the opposite end of cylinder 96 and support of filament 120 are accomplished by means of end disc 138. End disc 138 is brought to bear on 0 ring 108 by means of peripheral bolts and wing nuts 112 as before. In this case it should be noted, however, that flange 180 and end plate .138, when assembled, are not allowed to contact each other so as to remain electrically isolated by O ring 108. Filament fastening stud 140 is screwed axially into end plate 138 and anchors filament by means of set screw 144. In order to reduce heat losses from the filament, stud has neck 142 of reduced diameter between set screw 144 and end plate 138. End plate 138 is further provided with viewing aperture 146 covered by viewing glass v148 in order to permit accurate establishment of filament temperatures as by means of an external pyrometer (not shown). 0 ring .150 is sandwiched between end disc 138 and glass 148 to provide hermetic closure, the assembly being positioned by means of retainer ring 152. Once the internal temperatures are established as a function of current to filament 120 for a given apparatus and process, observation by aperture 146 is not required.

The reaction chamber thus described may be conveniently assembled and disassembled to permit ready replacement of the work or backing metal cylinder 154, which, with the apparatus of FIG. 3, comprises backing metal rolled into cylindrical form. Thermal insulation of backing metal cylinder .154 from reaction chamber cylinder 96 is conveniently achieved by means of cylindrical glass insert 156 into which the cylinder of backing material is conformably fitted.

It is .a feature of the invention that sorption of gas,

such as tritium, in a sorber film on a subtrate sheet may be accomplished in the apparatus of FIG. 2, using either the reaction chamber of FIG. 3 or that of FIG. 4, without complex auxiliary pumping systems for supplying the gas to be sorbed under pressure. Thus in one mode of operation, the gas to be sorbed may be supplied in a conventional 100 cc. flask at pressures of about 150 mm. Hg and delivered from the flask to the system. In systems in which the reaction chamber inner dimensions are approximately 12 inches in length and 1% inches in diameter and in which care has been taken to minimize the volume of the auxiliary piping so that the total system volume (valves 12, 15, 19, 32 and 34 closed) is of the order of 1 liter, a pressure of 15 millimeters may be achieved in reaction chamber 26 when starting with 150 mm. in the flask. By so minimizing auxiliary piping volume, a ratio of approximately 2 to 1 may be provided between the volumes of the reaction chamber and the remaining portions of the system, including manifold 8, coupling 24, and flask 28.

' It is another feature of the invention that contamination of newly formed sorber films is minimized by maximizing the area of newly formed sorber metal film rela tive to the total system volume. In this way, any residual gas remaining in the system after evacuation is thinly distributed and, therefore, when sorbed by the active surface of the newly formed film, results in low contamination per unit area. Thus, when processing foil areas of approximately 50 square inches, the system has an area to volume ratio of more than 1 square centimeter to 3 cubic centrimeters. By so minimizing contamination of newly formed films with undesirable nitrides, oxides, etc., the need for a processing step to disperse the superficial contamination through the body of the sorber film prior to sorption of the desired gas is eliminated. In addition, such contamination as does occur is so small that even if diffusion into the film is required in order to permit sorption of the desired gas, the quantity of desired gas which can subsequently be sorbed is not significantly reduced.

With the aforementioned apparatus, sorber metal coatings of uniform thickness suitable for tritiation may be produced over a backing metal area approximately 7 x 5 inches in size as follows.

A winding of pure sorber metal in wire form is evenly wound along the central 7 inch portion of a length of tungsten wire, to form the elongated, charge bearing filament. It has been found advantageous to electropolish the charge-receiving portion of the filament in a saturated solution of sodium carbonate so as to provide a surface which is readily wet by the sorber metal when it melts. The sorber metal charge wound on the filament should weigh, for example, .1 gram where sorber metal coatings of high adherence are desired. The .1 gram charge, may comprise 0.008 inch titanium wire wound with a pitch of & inch on the central 7 inch portion of a 3 strand 0.020 tungsten filament. This quantity of metal, when evaporated, is calculated to give films of titanium approximately 0.8 micron thick and films of zirconium approximately 0.6 micron thick over a 50 square inch area of foil. It will be understood that some variation in thickness in the film produced by evaporating the 7 inch length of charge occurs due to dispersion of the evaporating metal along the axis of the reaction chamber, with the result that films of substantially uniform thickness are produced over a length of substrate corresponding to the length of the charge and that beyond this central, seven inch portion of the substrate, the film thickness tapers off to substantially zero.

Where films of maximum thickness are required for the production of sources having maximum radiation per unit area, charge windings weighing approximately .15 gram are desirable. Such films have been found to have poor adherence characteristics, however.

Foil backing 4 (FIG. 1) may suitably comprise .002 inch type 302 stainless steel sheet electrolytically etched in a solution of one part of concentrated sulfuric acid, one part of 30% hydrogen peroxide and three parts of water. The etched surface so produced provides a base to which the vapor deposited sorber metal readily adheres. The dimensions of rectangular foil backing 4 should be approximately 6 by 12 inches, providing a 1 inch overlap when formed into a cylinder for insertion into a reaction chamber, yielding an active area of approximately 9 inches by inches. The overlap provides a convenient area for fastening the edges of the sheet together, as by spot welding, to prevent buckling of the cylinder during processing. The extra length of cylinder prevents the formation of tritiated films of substantial thickness on portions of the apparatus and in addition serves as a surface for receiving the aforementioned laterally dispersed sorber metal over an area useful in reducing the clean-up of residual gases by the central portion of the foil. This provides an approximately 40% increase in the sorber surface to system volume ratio over that which would be provided by a foil limited in length to correspond to the length of the charge on the filament.

Filaments and foil backings so prepared may be utilized in the reaction chambers of FIGS 3 and 4 interchangeably. Care should be taken during the loading of a reaction chamber to see that filament tensioning spring 86 (or 126) is sufficiently compressed to take up all slack in the filament which may occur during heating. Sagging of the filament with consequent unevenness of foil heating and coating or actual contact with the backing metal is thereby prevented.

With a loaded reaction chamber 26 and flask 28 containing 50 curies of tritium connected to the system of FIG. 2, the system is evacuated until a vacuum of less than .05 micron is obtained as indicated by vacuum gauge 18. The connection between valve 20 and tritium flask 28 must be pumped out prior to closing valve 20 and breaking the seal on flask 28 to release the contained gas. The filament with its charge of sorber metal and the cylindrically disposed sheet of stainless steel backing metal are then outgassed by passage of a current of approximately 35 amperes from autotransformer 23 through the filament, heating it to a temperature of approximately 1l00 C. When the parts have been thus vacuum baked and outgassed as indicated by again obtaining a vacuum of less than .05 micron, the filament temperature is raised momentarily to approximately 2200 C. by operation of the autotransformer to melt the charge and cause it to flow and wet the filament. The filament is then turned off and the foil allowed to cool, following which the filament temperature is again raised to 2200 and is maintained there until the charge from the filament is fully evaporated onto the backing metal substrate. A filament current of 50 amperes for 20 seconds has been found satisfactory for this purpose. Current to the filament is then turned off, and valves 13 and 15 to vacuum pump 10 are closed. The break-off seal on the tritium flask 23 is then cor.- ventionally broken by means of an iron slug actuated by an external solenoid (not shown), admitting tritium to manifold 8. After noting the pressure on gauge 18, valve 12 is closed and valve 13 is opened admitting tritium to reaction chamber 26. If the admission of tritium to the system is prompt, that is, within a matter of minutes, sorption of tritium by a newly formed titanium sorber layer will begin immediately. If there is a delay in admitting tritium, contamination of the surface of the newly formed film by a few molecular layers of residual air or apor may occur.

contaminating layers formed on the surface of the freshly formed sorber metal by delay in initiating the tritiation step, which are found to impede sorption, may be dispersed into the sorber layer by means of radiant heat supplied by reheating the filament until sorption is initiated as indicated by a drop in pressure on gauge 16.

In any event, where the full quantity of tritium supplied from the cc. flask is not immediately absorbed by the titanium film, as may occur because of decreasing sorption rate with decreasing reaction chamber pressure, further heat may be supplied to the chamber by means of the filament to initiate a further sorption reaction. Surprisingly, it has been found that combination ratios considerably greater than 1.0 can consistently be achieved with sorption pressures as low as 2 mm. of Hg in the reaction chamber. This performance of the system is at variance with published data such as that reproduced in FIG. 1.10.7 at page 167 of The Reactor Handbook, United States Atomic Energy Commission, March 1955, AECD- 3647, which indicates that pressures above 7 millimeters are required in order to obtain combination ratios above 0.8. Without wishing in any way to be bound by it, it is suggested that this unexpected behaviour may be due, not only to the supplemental supply of heat which is given to the reaction by further filament heating, but also to the presence of ionized tritium produced by heat from the filament. Regardless of the theory, it has been found that essentially complete tritiation of films of desired thicknesses can be achieved which substantially utilizes the volume of tritium gas supplied.

In a second mode of operation, quantities of tritium gas in excess of that supplied in any one 50 curie, 100 cc. flask may be obtained by storing the contents of more than one flask in uranium trap 30. Loading of uranium trap 30 may be accomplished in advance of connection of reaction chamber 26 to manifold 8. To do this, valves 12 and 13 are closed to shut off reaction chamber 26 and manifold 8 and the space between flask 28 and valve 20 evacuated through valve 15 until vacuum gauge 18 shows a vacuum of 0.1 micron or less. Valve 15 is then closed and the glass flask tip-off (not shown) of gas storage flask 28 broken, and valve 32 to uranium trap 30 opened. Heat from coil 40 is then supplied to uranium trap 30 to initiate the sorbing action of the uranium in the trap. Upon completion of sorption of the tritium thus released into manifold 8, valve 32 to uranium trap 30 and valve 20 to tritium flask 28 are closed and a second tritium flask inserted in quick coupling 24. The operation is then repeated, valve 20 being opened to pump down quick coupling 24 prior to breaking the tritium flask seal. Tritium may thus be stored in trap 30 in a quantity limited only by the sorb ing capacity of the contained uranium. The use of trap 30 in this way also has the advantage of further improving the ratio between re action chamber volume and system volume, since the volume of flask 28 need not be connected to the system during tritiation of a foil. Finally, since more than one 50 curie volume of tritium can be contained in the trap, higher pressure may be achieved in the system, pressures which notably assist in the tritiation of some materials. Without in any way departing from the essential teachings and spirit of the invention, the methods and apparatus may be modified to accommodate other modes of operation, as will be apparent to those skilled in the art. For example, cleaning and outgassing of the parts may be accomplished by gaseous discharge, rather than vacuum baking as outlined above, by pre-assembly bakeout of charge, filament and substrate, or by a combination of these methods. The reaction chamber of FIG. 4 may be adapted for gaseous discharge cleaning by passing a hermetically sealed conductor through sight glass 148 and providing a conventional electrical connector for attachment between the conductor and the cylindrical substrate. By means of an electrical field impressed between the filament and the substrate, an inert gas, such as helium, admitted to the reaction chamber through auxiliary manifold connection 36 may be ionized. Bombardment of the filament, charge, and substrate by the ions so produced will thus produce the desired cleaning operation.

It will be understood that the sheet substrate, coated with tritiated sorber metal, is removed from the apparatus after tritiation, and unrolled. When unrolled, it may be trimmed to remove inactive marginal regions and used in one piece or it may be cut to provide pieces of varying smaller sizes.

While the foregoing discussion has revolved around the preparation of thin tritiated films of titanium or zirconium, it will be apparent to those skilled in the art that the same techniques and apparatus may be utilized in forming films of other metals such as beryllium, lithium, columbium, etc., and that gases other than tritium, such as hydrogen, deuterium, oxygen and nitrogen, may readily be employed. The following claims should therefore be construed in scope commensurate with the spirit of the invention.

I claim:

1. A method of preparing a substrate clad with a metal containing sorbed gas which includes the steps of placing said substrate in an evacuable chamber with the surface to be coated exposed to a heat source carrying a charge of sorber metal, evacuating said chamber and maintaining the vacuum so produced while performing the steps of heating said source to a temperature below the melting point of said charge to outgas said charge and said surface, heating said source to a higher temperature to melt said charge on said source, heating said source to cause evaporation of said charge and formation of a layer of sorber metal on said substrate surface, and immediately upon the formation of said layer of sorber metal admitting a sorbable gas to the still evacuated chamber to cause sorbtion thereof in the newly formed layer on said substrate.

2. The method of claim 1 including the step of establishing a ratio between the area of sorber metal evaporated onto said substrate and the volume of evacuated space within said chamber which is at least proportional to one unit of area in centimeters per three units of volume in centimeters.

3. A method of preparing a substrate clad with a metal containing an isotope of hydrogen which includes the steps of etching a surface of a substrate, placing said substrate in an evacuable chamber with the etched surface exposed to a heat source carrying a charge of sorber metal, evacuating said chamber and maintaining the vacuum so produced while performing the steps of heating said source to a temperature below the melting point of the charge to outgas said charge and said surface, heating said source to a higher temperature to melt said charge on said source, heating said source to cause evaporation of said charge and formation of a hot layer of sorber metal on said surface, and promptly thereafter admitting an isotope of hydrogen gas to the still evacuated chamber to cause sorption thereof in the newly formed hot layer on the substrate as it cools.

4. The method of claim 3 in which supplemental heat is supplied to said substrate from said heat source, as required, to further sorption of said gas.

5. A method of preparing a metal substrate clad with a metal containing an isotope of hydrogen which includes the steps of etching a surface of said substrate, placing said substrate in an evacuable chamber with the etched surface exposed to a heat source carrying a charge of sorber metal, evacuating said chamber and maintaining the vacuum so produced while performing the steps of heating said source to a temperature below the melting point of said charge to outgas the charge by conduction heat and said substrate by radiant heat, heating said source to a higher temperature to melt said charge on said source, heating said source to cause evaporation of the charge and formation of a layer of sorber metal on said surface, and promptly thereafter admitting an isotope of hydrogen to the still evacuated chamber to cause sorption thereof in the newly formed hot layer on said substrate.

6. A method of cladding a surface of a sheet metal substrate with a metal containing a sorbed gas which in cludes the steps forming said substrate into a hollow cylinder, mounting said cylinder in an evacuable chamber having a conforming support of low thermal conductivity for said substrate, coaxially mounting a narrow, elongate heat source carrying a length of evenly distributed sorber metal of known weight within said cylindrically disposed substrate, evacuating said chamber and maintaining the vacuum while performing the steps of heating said source to a temperature below the melting point of said sorber metal to outgas said source, said sorber metal, and said substrate, heating said source to a higher temperature to melt said sorber metal on said substrate, heating said source to cause evaporation of said charge and formation of a hot film of controlled thickness on said substrate, and promptly thereafter admitting a sorbable gas to the still evacuated chamber to cause sorption thereof in the newly formed hot layer on said substrate.

7. The method of claim 6 including the steps of etching the surface of said substrate to be clad and forming the substrate into a cylinder with the etched surface facing inward towards the heat source.

8. The method of claim 6 including the steps of evenly distributing the sorber metal along the length of said elongate heat source by winding a Wire of sorber metal thereon.

9. The method of claim 6 including the step of establishing a ratio between the area of sorber metal evaporated onto said substrate and the volume of evacuated space within said chamber which is at least proportional to one unit of area in centimeters per three units of volume in centimeters.

10. The method of preparing a substrate clad with a metal containing a sorbed gas which comprises heating and evaporating in an evacuable chamber under a predetermined subatmospheric pressure a degassed charge of sorber metal onto a surface of degassed substrate and there form a hot layer of sorber metal, and promptly thereafter admitting a sorbable gas to the still evacuated chamber to expose said newly formed hot layer of sorber metal to said gas to cause sorption of said gas therein.

11. The method of preparing a substrate clad with a metal containing an isotope of hydrogen which comprises heating and evaporating in an evacuable chamber under a predetermined subatmospheric pressure a degassed charge of sorber metal selected from the group consisting of titanium and zirconium onto a surface of a degassed substrate and there form a hot layer of sorber metal, and promptly thereafter admitting an isotope of hydrogen to the still evacuated chamber to expose said newly formed hot layer of sorber metal to said gas to cause sorption of said gas therein.

References Cited in the tile of this patent UNITED STATES PATENTS 1,602,634 Anderson Oct. 12, 1926 1,647,618 Gustin Nov. 1, 1927 2,004,580 Meyer June 11, 1935 2,041,569 Reerink et a1 May 19, 1936 2,267,343 Scott et a1 Dec. 23, 1941 2,450,340 Hensel et a1. Sept. 28, 1948 2,635,994 Tier-man Apr. 21, 1953 2,701,849 Penning et a1. Feb. 8, 1955 12 Dewan et a1 Feb. 14, 1956 Cook et a1. Oct. 23, 1956 Bjorksten Apr. 16, 1957 Marvin May 13, 1958 Ashley Aug. 12, 1958 Schwindt May 12, 1959 Sheppard Nov. 8, 1960 OTHER REFERENCES 10 Johnson, H. C. E.: Scientific American, vol. 176, N0. 3,

March 1947, pp. 116, 117, 118.

Graves et al.: Review of Scientific Instruments, vol. 20, No. 8, August 1949, pages 579-582.

Gulbransen, E, A., and Andrew, K. F.: Transactions of 15 the Electrochemical Society, vol. 96, No. 6, December Massey: Preparation of Thin Tritium Zirconium Targets, ORNL-2237, Feb. 21, 1957, 8 pages. 

1. A METHOD OF PREPARING A SUBSTRATE CLAD WITH A METAL CONTAINING SORBED GAS WHICH INCLUDES THE STEPS OF PLACING SAID SUBSTRATE IN AN EVACUABLE CHAMBER WITH THE SURFACE TO BE COATED EXPOSED TO A HEAT SOURCE CARRYING A CHARGE OF SOBER METAL, EVACUATING SAID CHAMBER AND MAINTAINING THE VACCUUM S PRODUCED WHILE PERFORMING THE STEPS OF HEATING SAID SOURCE TO A TEMPERATURE BELOW THE MELTING POINT OF SAID CHARGE TO OUTGAS SAID CHARGE AND SAID SURFACE, HEATING SAID SOURCE TO A HIGHER TEMPERATURE TO MELT SAID CHARGE ON SAID SOURCE, HEATING SAID SOURCE TO CAUSE EVAPORATION OF SAID CHARGE AND FORMATION OF A LAYER OF SORBER METAL ON SAID SUBSTRATE SURFCACE, AND IMMEDIATELY UPON THE FORMATION OF SAID LAYER OF SORBER METAL ADMITTING A SORBABLE GAS TO THE STILL EVACUATED CHAMBER TO CAUSE SORBTION THEREFORE IN THE NEWLY FORMED LAYER ON SAID SUBSTRATE. 